The present invention relates to an irradiation system, and more particularly to a system having a compact shield arrangement for containing radiation within the system to ensure the safety of operating personnel.
Irradiation technology for medical and food sterilization has been scientifically understood for many years dating back to the 1940""s. The increasing concern for food safety as well as safe, effective medical sterilization has resulted in growing interest and recently expanded government regulatory approval of irradiation technology for these applications. The available sources of ionizing radiation for irradiation processing consist primarily of gamma sources, high energy electrons and x-ray radiation. The most common gamma source for irradiation purposes is radioactive cobalt 60 which is simple and effective but expensive and hazardous to handle, transport, store and use. For these reasons, electron beam and x-ray generation are becoming the preferred technologies for material irradiation. An exemplary maximum electron beam energy for irradiation purposes is on the order of 10 million electron-volts (MeV) which results in effective irradiation without causing surrounding materials to become radioactive. The necessary electron beam power must be on the order of 5 to 10 kilowatts or more to effectively expose materials at rates sufficient for industrial processing.
Electron beam and x-ray irradiation systems both employ an electron accelerator to either emit high velocity electrons directly for irradiation or to cause high velocity electrons to collide with a metal conversion plate which results in the emission of x-rays. A number of electron acceleration techniques have been developed over the past several decades including electrostatic acceleration, pumped cylindrical accelerators and linear accelerators.
Electrostatic accelerators are characterized by the use of a direct current static voltage of typically 30 to 90 kilovolts which accelerates electrons due to charge attraction. Electrostatic accelerators are limited in maximum energy by the physical ability to generate and manage high static voltage at high power levels. Electrostatic accelerators using Cockroft-Walton voltage multipliers are capable of energy levels of up to 1 MeV at high power levels, but the 10 MeV energy level utilized by many systems for effective irradiation is not typically available.
Various types of pumped cylindrical electron beam accelerators have been known and used for many years. These accelerators generally operate by injecting electrons into a cylindrical cavity, where they are accelerated by radio frequency energy pumped into the cylinder. Once the electrons reach a desired energy level, they are directed out of the cylinder toward a target.
RF linear accelerators have also generally been in use for many years and employ a series of cascaded microwave radio frequency tuned cavities. An electron source with direct current electrostatic acceleration injects electrons into the first of the cascaded tuned cavities. A very high energy radio frequency signal driven into the tuned cavities causes the electrons to be pulled into each tuned cavity by electromagnetic field attraction and boosted in velocity toward the exit of each tuned cavity. A series of such cascaded tuned cavities results in successive acceleration of electrons to velocities up to the 10 MeV level. The accelerated electrons are passed through a set of large electromagnets that shape and direct the beam of electrons toward the target to be irradiated.
A typical industrial irradiation system employs an electron beam accelerator of one of the types described, a subsystem to shape and direct the electron beam toward the target and a conveyor system to move the material to be irradiated through the beam. The actual beam size and shape may vary, but a typical beam form is an elliptical shape having a height of approximately 30 millimeters (mm) and a width of approximately 45 mm. The beam is magnetically deflected vertically by application of an appropriate current in the scan deflection electromagnets to cause the beam to traverse a selected vertical region. As material to be irradiated is moved by conveyor through the beam, the entire volume of product is exposed to the beam. The power of the beam, the rate at which the beam is scanned and the rate that the conveyor moves the product through the beam determines the irradiation dosage. Electron beam irradiation at the 10 MeV power level is typically effective for processing of food materials up to about 3.5 inches in thickness with two-sided exposure. Conversion of the electron beam to x-ray irradiation is relatively inefficient but is effective for materials up to 18 inches or more with two-sided exposure.
In electron beam irradiation, high energy electrons are directed toward various food products which cause secondary radiation to be generated to penetrate deeply within the product. A byproduct of this beneficial secondary radiation is the generation of potentially harmful scattered radiation in the area of the system while it is operating. Consequently, radiation shielding is necessary to insure the safety of operating personnel.
Shielding requirements are determined by the power and the energy of the radiation source. Energy is related to the velocity of the accelerated electrons and generally determines penetration capability. Power is related to the number of accelerated electrons and generally determines exposure rate capability. For personnel safety, both parameters must be considered, as each contributes to the ability of radiation to penetrate shielding structures in amounts that must be limited for safe long term exposure by humans.
Electron beam irradiation systems maybe designed at various power and energy levels, with the maximum allowable energy established by the FDA and USDA at 10 MeV. This level has been selected as an upper bound due to the fact that no materials are activated and rendered radioactive by exposures at or below this level. While useful irradiation processing may be performed with electron beam energies as low as 1 MeV, the penetration depth that is possible at such energies is well less than 0.3 inches and is therefore limited in application. Energies of 10 MeV, however, allow two sided penetration up to 3.5 inches, which is useful for a wide variety of food products in final packaging. The disadvantage of the higher irradiation energy sources is the fact that shielding requirements are substantially greater to insure the safety of operating personnel. Typical 10 MeV systems are constructed within entire special buildings constructed of continuously poured high density concrete that may be as thick as 10 feet. Materials are typically moved to the radiation source by a conveyor that moves around a maze structure in circuitous fashion to insure that there is no straight line path for radiation to escape. While such structures are effective in providing safe operating conditions, they are also expensive and inefficient to construct and operate, and are difficult to add to an existing facility or production system.
There is a need in the art for an irradiation system employing a compact and yet effective shielding system for containing irradiation and preserving the safety of operating personnel. Such a system is the subject of the present invention.
The present invention is an irradiation system that includes a radiation source providing radiation in a localized radiation exposure area and a shielding structure around the radiation source. A conveyance system transports product into the shielding structure, through the radiation exposure area and out of the shielding structure. The conveyance system includes an input portion for carrying the product into the shielding structure at a first elevation. A first elevator moves the product from the first elevation to a second elevation different from the first elevation. A processing portion of the conveyance system carries the product at the second elevation through the radiation exposure area. A second elevator moves the product from the second elevation to a third elevation different from the second elevation. An output portion of the conveyance system carries the product out of the shielding structure at the third elevation. This configuration allows the shielding structure to have a reduced size in comparison to prior art irradiation systems, so that the irradiation system may be more easily added to existing processing facilities, or simply installed in a smaller area. The compact shielding structure may also be built for less cost than the concrete bunker required by many prior art irradiation systems.
In another embodiment, the conveyance system includes first and second input portions for carrying the product into the shielding structure and first and second output portions for carrying the product out of the shielding structure. A first elevator moves the product from the first input portion at first elevation to a second elevation different from the first elevation, and a second elevator moves the product from the second input portion at the first elevation to the second elevation. A first transfer portion of the conveyance system controllably transfers the product from the first and second elevators to a processing portion of the conveyance system for carrying the product through a localized radiation exposure area, and on to a second transfer portion of the conveyance system for controllably passing the product. A third elevator moves the product passed from the second transfer portion at the second elevation to the first output portion of the conveyance system at a third elevation different from the second elevation, and a fourth elevator moves the product passed from the second transfer portion at the second elevation to the second output portion of the conveyance system at the third elevation. This embodiment potentially increases the throughput of the irradiation system, and provides a redundant product path to reduce down time in the case of failure of one of the input portions or output portions of the conveyance system.